1. Field of the Invention
This invention relates generally to a hydrostatic axial piston machine utilizing a swash plate construction having a plurality of pistons mounted so that they can move axially in a cylinder drum, each piston being supported on a slide face and hydrostatically compensated by an individual sliding block having a sliding block plate facing the slide face and which is connected to the piston on the side opposite the sliding block plate, with a sealing web formed on the sliding block plate and with the sliding block provided with a bore extending from the piston side of the sliding block to the slide face side.
2. Description of the Currently Available Technology
In known swash plate machines, the pistons are usually each supported by means of an individual sliding block on a slide face to reduce the friction forces between the pistons and the swash plate and thus to increase the efficiency of the machine. The slide face can be formed by a stationary or tilting swash plate and/or by a wear plate that is non-rotatably connected to the swash plate. Hydrostatic compensation occurs by means of a passage or bore in the sliding blocks, which bore extends from the area in contact with the piston to the area of the sliding block plate. The bore is in flow communication with a longitudinal bore in the piston through which hydraulic fluid can flow from the piston to the sliding block plate.
On a swash plate machine used as a hydraulic motor, there are stringent requirements in terms of the uniformity of the motion, primarily during start-up from a stop and during operation at low speeds. With a constant flow of hydraulic fluid flowing to the hydraulic motor, there will only be a uniform rotational movement if the change in the leakage at the sliding blocks is as small as possible over the angle of rotation of the hydraulic motor.
In known axial piston machines, the sliding block plate of the sliding block is flat. The sliding block plate has a pressure pocket that is in communication with the bore and has a sealing web that concentrically surrounds the pressure pocket. The sealing web, which simultaneously forms the bearing surface of the sliding block, is therefore flat and, in the unloaded condition, is oriented parallel to the slide face. To achieve a correspondingly low leakage at the sliding blocks, the sealing web of the sliding block is realized so that it is correspondingly wide.
During operation of a swash plate machine, friction occurs at the connecting point between the piston and the sliding block, e.g., a ball bearing, which generates a tilting moment that is exerted on the sliding block. This tilting moment can result in an inclined position of the sliding block with respect to the wear plate, which has an adverse effect on the quality of the seal and the magnitude of the friction on the sliding block. During the work stroke of the piston, the length of the piston that projects out of the cylinder drum also changes, as a result of which the support forces of the piston in the cylinder guide and the associated friction forces also change. The normal force applied to the sliding block is therefore not constant and changes over the stroke of the piston.
Under these conditions, a sliding block with a flat bearing surface behaves in the following manner. As a result of the tilting moment acting on the sliding block, the sliding block assumes a position in which it is tilted with respect to the slide face. The tilting of the sealing web of the sliding block with respect to the slide face results in an asymmetrical pressure profile under the sealing web, which results in a hydrostatic righting moment on the sliding block that is opposite to the tilting moment. Consequently, there are measures to counteract an increase in the leakage of hydraulic fluid and a tilting of the sliding block and the associated increase in the friction forces.
With such sliding blocks having a flat bearing surface and thus a flat sealing web, if the normal force pressing on the sliding block during a stroke of the piston decreases, the distance between the bearing surface of the sliding block and the slide face increases, as a result of which the height of a sealing gap between the sliding block and the slide face increases. The pressure profile in the sealing gap thereby changes only slightly over the height of the sealing gap, as a result of which the hydrostatic compensation force remains practically constant in spite of the change in the height of the sealing gap. As the height of the sealing gap increases, there is therefore a significant increase in the leakage between the sliding block and the slide face. Over the piston stroke, therefore, there is only one point at which the compensation of the sliding block specified by the designer can be achieved. At all the other operating points, the sliding block is either under-compensated, as a result of which the sliding block is pressed against the slide face and thus the friction increases, or the sliding block is over-compensated, as a result of which the sliding block lifts up away from the swash plate and the height of the sealing gap increases accordingly. The result is increased leakage, which leads to high leakage oil pulsation of the machine. As a result of this fluctuation of the leakage oil flow and the friction forces during one revolution of the cylinder drum, there is a non-uniform rotational movement of the machine.
To reduce this leakage oil pulsation, hold-down devices are required on the sliding blocks. However, such hold-down devices require a correspondingly precise adjustment and are expensive to manufacture. The leakage oil pulsation can be reduced by using a spring device to exert additional pressure on the sliding block, but that also results in increased friction.
A longitudinal section of a known axial piston machine in a conventional swash plate design used as a hydraulic motor is shown in FIG. 1. A cylinder drum 1 drives a connected output shaft 2. The cylinder drum 1 has a plurality of bores 3 located concentric to an axis of rotation 11 of the output shaft 2. A piston 4 is mounted in each bore 3 so that it can move longitudinally with respect to the cylinder drum 1. The pistons 4 are provided, on the end projecting out of the bore 3, for example, with a spherical head 5 which engages a spherical recess formed on a sliding block neck of a sliding block 6. The sliding block 6, on the side opposite the piston 4, has a sliding block plate 7 which is configured to abut a slide face 8, e.g., of a wear plate 12 that is fastened non-rotationally to a swash plate. A bore 9 located in the sliding block 6 is in flow communication with a longitudinal bore 10 in the piston 4, whereby hydraulic fluid can flow from the pressure chamber formed by the bore 3 and the piston 4 to the sliding block plate 7 and thus the piston 4 is hydrostatically compensated on the slide face 8.
FIG. 2 shows a detail of the sliding block 6 of FIG. 1. On the sliding block plate 7, there is a ring-shaped pressure pocket 16 that is in flow communication with the bore 9 oriented coaxially to an axis of symmetry 18. A sealing web 15, realized in the form of a flat surface, concentrically surrounds the pressure pocket 16. The width of the sealing web 15 is defined by the outside diameter d.sub.a and the inside diameter d.sub.i of the sealing web 15.
During the operation of an axial piston machine used as a hydraulic motor, a load is applied to the sliding block 6 in the form of a normal force F.sub.N which changes over the stroke of the piston 4. As a result of the application of the normal force F.sub.N, friction occurs at the ball-and-socket joint between the piston 4 and the sliding block 6, which causes a tilting moment M.sub.R that acts on the sliding block 6 and causes it to assume a position that is tilted at an angle .gamma. with respect to the slide face 8. The pressure profile 17 illustrated in the bottom portion of FIG. 2 thereby occurs on the sliding block 6. As a result of the inclined position of the sliding block 6 with respect to the slide face 8 and the resulting different distance of the sealing web 15 from the slide face 8, an asymmetrical pressure profile 17 is thereby formed under the sealing web 15. The result is a hydrostatic compensation force F.sub.E that is applied at a distance e from the axis of symmetry 18 of the sliding block 6. The compensating force F.sub.E therefore exerts a righting moment on the sliding block 6 that counteracts the tilting moment M.sub.R. A sealing gap having a height s is defined between the slide face 8 and the sealing web 15.
It is an object of this invention to provide a hydrostatic axial piston machine that has low or reduced friction between the sliding blocks and the slide face and a low leakage oil pulsation as a result of the leakage at the sealing webs.